It is known that cold methanol has the capability of absorbing large amounts of gas impurities. Use is made of this capability in the Rectisol® process, in which the total purification of compressed gases is possible in a single process operation. The absorptive capability of methanol increases considerably at lower temperatures. At −60° C. for example 75 times more carbon dioxide dissolves in methanol than in the same volume of water at 25° C., that is the methanol cycle amounts to 1/75 compared with water recirculation in a pressurized water cycle. At lower temperature the vapor pressure of methanol is so low that there is little use of solvent.
The Rectisol® process is particularly efficient when large amounts of gas impurities must be removed, or if a particularly high gas purity is required, and overall where the process can be built into the cold stage of a low temperature gas decomposition. In the latter case the process itself offers important advantages when only small quantities of gaseous impurities are to be washed out.
Gas washing is carried out such that the standing raw gas is treated with methanol under medium pressure of 5 to 40 atmospheres or also under high pressure of 50 to 200 atmospheres at temperatures between 10° C. and −80° C.
This way, all gaseous impurities such as raw gasoline, crude benzene, ammonia, hydrocyanic acid, resin formers, organic sulfur and phosphorus compounds, carbonic acid, hydrogen sulfide, iron and other metal carbonyls and water are absorbed. The loaded-up methanol is then regenerated by expansion, evaporation, or heating, and subsequently re-used. The impurities can be recovered from the off-gases or condensate. This process is the subject of German patent 1 544 080 (U.S. Pat. No. 3,453,835).
An especially important gas purification process known in the art includes purification of, for example, synthesis gases produced from natural gas gasification processes, which are the starting point for a variety of major technical syntheses. Raw synthesis gas contains considerable amounts of carbon dioxide whose removal is crucially important for the further use of synthesis gasses. The development of effective, reliable, and cost-effective processes for removal of carbon dioxide from synthesis gasses is therefore of considerable importance for the efficient production of a variety of applicable gas mixtures.
It has now been found that the process known up until now for the purification of gases with methanol may still be improved considerably, if the system in accordance with the invention, and the process that may be carried out therein for desorption of carbon dioxide, is employed.